A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to ensure proper operation of the integrated circuits that are manufactured by the successive patterning process (note that an IC may comprise up to 30 or more successive layers) an accurate alignment of successive layers needs to be performed. An important condition to realize such an accurate alignment is to make sure that a positioning of the substrate relative to the patterning device (or reticle) is controlled as accurately as possible. To facilitate this, vibrations of the structures supporting the substrate or patterning device, the projection system or the illumination system should be avoided or suppressed.
During a typical exposure cycle, both the substrate and the patterning device undergo significant accelerations. In order to generate such accelerations, powerful positioning devices, typically including a plurality of electromagnetic actuators, linear or planar motors, are applied to exert the appropriate forces to the support structures of the substrate and the patterning device. As will be understood by the skilled person, when acceleration or deceleration forces are applied to such support structures, equal and opposite reaction forces are generated as well. When such reaction forces would be exerted on a stationary frame of the lithographic apparatus, significant vibrations would be generated throughout the apparatus, thus adversely affecting the accuracy of the exposure process. In order to avoid the generation of such vibrations, a balance mass arrangement is typically applied, whereby the reaction forces are not applied to a stationary frame but to a counter mass which is configured to displace in a substantially frictionless manner relative to the stationary frame or frames of the apparatus. By using such a balance mass arrangement, the direct application of significant reaction forces on the lithographic apparatus can be avoided. However, it has been found that such a balance mass arrangement may also be the cause of torque components exerted on the stationary frames and structures of the lithographic apparatus. In particular, it has been found that the illuminator or illuminator module of a lithographic apparatus is susceptible to such components, and may thus suffer from vibrations caused by such torque components. As a result, the illuminator or illuminator module may displace relative to the patterning device, the projection system or the substrate, adversely affecting the exposure process.